Catalysts were prepared by the condensation reaction of tetraethoxysilane with the surface of alumina and characterized by X-ray photoelectron spectroscopy to determine surface compositions and by Infrared spectroscopy with adsorbed pyridine to determine surface acidity. Silicon-containing groups formed on the alumina surface at less than monolayer coverage, inducing strong acidity; both Bronsted and Lewis acid groups were detected. The catalytic activity for skeletal isomerization of 1-butene was determined In flow reactor experiments at atmospheric pressure and 450-525 °C. The reaction is first-order In the reactant; the activity increased with Increasing surface acidity, i.e., with increasing Si content of the surface and apparently with increasing Bronsted acidity. The surface is comparable in structure and catalytic activity to that of silica-alumina, and the catalytic sites are suggested to be Si-OH groups positioned next to coordinatively unsaturated Al3+ sites.